Using a sharp kitchen knife, cut a small piece from the bottom of each grapefruit half, so that the flat base of the fruit will sit comfortably on the rack of your broiler pan.
Preheat the broiler. Take a grapefruit or other serrated knife and carefully cut around each grapefruit half, gently loosening the flesh from the skin. Then cut between the segments, so that they will be easy to spoon out.
Spoon the honey onto the top of each grapefruit half, pressing gently so that it oozes through the segments.
Arrange the grapefruit halves on the grill rack, and broil under a medium heat for about 2 minutes.
Sprinkle over the sesame seeds and broil for a further 30 seconds (no longer, or the seeds will burn). Serve immediately.